There is known a receiver which combines several components of multi-path signals that are mutually delayed by different time delays before reaching the receiver. Such a receiver is for example present in code division multiple access (CDMA) wireless communication systems and is currently designated by the name of Rake-type receiver or “Rake” receiver.
In a wireless communication system, a base station communicates with a plurality of remote terminals, such as cellular mobile telephones. Frequency division multiple access (FDMA) and time division multiple access (TDMA) are the conventional multiple access systems for delivering simultaneous services to a certain number of terminals. The basic idea underlying the FDMA and TDMA systems is dividing the available resource into several frequencies or into several time slots, respectively, such that several terminals can operate simultaneously without causing interference.
Telephones operating according to the GSM standard belong to the FDMA and TDMA systems in the sense that transmission and reception take place at different frequencies and also in different time slots.
Unlike these systems using frequency division or time division, CDMA systems enable multiple users to share a common frequency and a common time channel by using a coded modulation. Amongst the CDMA systems are the CDMA 2000 system, the WCDMA system (wide band CDMA), and the IS-95 standard.
In CDMA systems, a scrambling code is associated with each base station and is used to distinguish one base station from another. In addition, an orthogonal code, known as the OVSF code, is allocated to each remote terminal (such as a cellular mobile telephone). All the OVSF codes are mutually orthogonal which distinguishes one channel from another.
Before transmitting a signal over the transmission channel to a remote terminal, the signal has been scrambled and spread by the base station using the scrambling code of the base station and the OVSF code of the channel.
In CDMA systems, those that use a distinct frequency for transmission and reception (CDMA-FDD system) can be distinguished from those which use a common frequency for transmission and reception, but distinct time domains for transmission and reception (CDMA-TDD system).
The present invention applies advantageously to communication systems of the CDMA type, and is particularly suited to systems of the WCDMA type with terrestrial radio access (UTRA FDD/TDD).
The incident signal received by a mobile telephone for example comprises different versions delayed in time from the signal initially transmitted, versions or echoes which are the result of the multi-path transmission characteristics of the transmission environment between a base station and the telephone, with each path introducing a different delay.
The “Rake” receiver in a cellular mobile telephone operating in a CDMA communication system is used to carry out temporal alignment, descrambling, compression, channel correction and combination of the delayed versions of the initial signals in order to deliver the information streams (symbols) contained in the initial signals.
A “Rake” receiver comprises several fingers, with each finger being intended to demodulate a given path received at a given instant.
Furthermore, the receiver comprises a channel estimation unit the purpose of which is to identify the various echoes through their delay and their mean energy as well as a mechanism for selecting echoes with a view to their respective assignment to the fingers of the Rake receiver.
Generally, the number of echoes detected is greater than the processing capacity of the receiver (number of fingers) for hardware complexity limitation reasons.
Hence, at the present time use is made of a selection mechanism capable of selecting from among the L echoes detected the N best echoes (N being the number of fingers of the receiver), that is to say those that provide the minimum binary error rate.
The criterion generally adopted is that of the signal-to-noise ratio. Thus, the N echoes with the largest signal-to-noise ratio are selected from among the L echoes detected.
Such a criterion provides a good solution as long as the minimum separation between the various echoes of the radio propagation profile (channel estimation) remains relatively sizeable. To increase the performance of the receiver, it is advisable to take account of close echoes, that is to say temporally correlated echoes.